Z01 HD 01123-10 LCE and Z01 HD 01124-10 LCE are companion projects that together investigate auditory communication in primates. The overall goal of these studies is to provide a comprehensive understanding of primate auditory communication in terms of development, neural mechanisms, endocrine factors, and social context. Two non-human primates, the squirrel monkey and common marmoset, are the main subjects of study, with additional data collected from other species where appropriate. Prior work in this project has shown that production of sounds that are the functional and acoustic equivalents of cry sounds in human infants are mediated by ?limbic? cortex located along the anterior midline of the frontal lobe of the cerebral cortex, and that single neural elements in the auditory cortex (superior temporal gyrus) are particularly responsive to subtle differences in the acoustic structure of species-specific vocalizations, suggesting an important role in mediating individual differences (?vocal signatures?). Studies of endocrine mechanisms have shown that endogenous prolactin levels correlate with carrying time in infant retrieval tests, and that reducing prolactin by administering bromocryptine (a dopamine agonist) disrupts infant retrieval. New findings for FY2000 are (a) play behavior in juvenile marmosets is increased by administering the opiate agonist morphine sulfate peripherally, and decreased by the opiate antagonist naloxone; (b) prolactin levels of adult female squirrel monkeys are significantly associated with amount of affiliative behavior that they engage in with infants in their group (where the females serve as ?aunts? to the infants born to other females in the group); (c) a new model of brain organization, the ?communicating brain,? posits that a suite of neural structures found in all land vertebrates and shown to be directly involved in vocal production have experienced a conservative evolutionary history, and mediate the same functional categories of vocalizations in a wide range of species, including humans.